1. Field of the Invention
This invention relates to a dc-dc converter for a video apparatus such as a portable video camera.
2. Description of the Prior Art
A dc-dc converter is used in a portable apparatus such as a video camera to obtain a higher voltage necessary for the portable apparatus from a relatively low voltage power supply, such as a battery. Moreover, in the stationary type apparatus such as VTR, a dc-dc converter is used in a switching regulator for producing a necessary dc voltage after converting an ac voltage from a commercial power supply to a dc voltage in order to miniaturize a power supply unit and reduce the weight of the power supply unit.
FIG. 4 is a block diagram of a prior art dc-dc converter.
In the prior art dc-dc converter 100, a self-excited oscillation circuit 101 produces and sends a square wave signal 102 to a clock input terminal 103a of a pulse width modulation circuit 103. The pulse width modulation circuit 103 produces a pulse width modulation signal 105 whose duty ration is changed but whose cycle is maintained, in accordance with a control voltage 104 applied to a duty control terminal 103b. The pulse width modulation signal 105 causes switching of a power FET or a transistor in the drive circuit 106 to start and stop a current flow in the primary circuit of the transformer 107. An output of a secondary circuit of the transformer 107 is sent to a rectifying and smoothing circuit 108 for obtaining a dc voltage output 109.
The dc voltage output 109 is fed back to the above-mentioned voltage control terminal 103b directly. Alternatively, a voltage corresponding to the dc voltage output 109 is fed back to the voltage control terminal 103b indirectly to stabilize the dc voltage output 109. That is the dc voltage output 109 is sent to the control terminal 103b after voltage-dividing.
In such prior art dc-dc converter, there is a problem that if noise developed by switching is imposed upon a video signal, the noise affects the reproduced image. If the occurrences of the noise synchronizes with the horizontal scanning signal, a vertical stripe pattern occurs on the reproduced image.
Moreover, another type dc converter is known. That is, a control input terminal 101a for stopping oscillation is provided in the self-excited oscillation circuit 101. Its power consumption is reduced by stopping of dc-dc converting by controlling of a logic level of an oscillation control signal 110 applied to the control input terminal 102a when the apparatus is in the waiting state.
However, the prior art dc-dc converter 100 shows a low stability because it uses an LC type or CR type self-excited oscillating circuit 101 to make the structure simple and reduce its cost. Moreover, the square wave signal 102 of the self-excited oscillation circuit 101 is not synchronized with a vertical synchronizing signal of the video signal system. Therefore, there is a problem of beat occurring on a reproduced image due to noise developed by switching because the effects of such noise cannot removed totally even though oscillation frequency of the self-excited oscillation circuit 101 is adjusted.
In order to resolve such problems, this invention is provided to reduce the beat trouble associated with switching noise caused by a dc-dc converter with a simple structure if an oscillating circuit having a relatively low stability.